In a number of industrial processes a relatively cool feed fluid is fed into a vessel and mixed with the hotter fluid therein. Examples include heat exchangers and steam generators. For example, in a steam generator such as a boiling water nuclear reactor the heat source is a nuclear fuel core contained in a pressure vessel. Typically, a feedwater pipe is connected to the pressure vessel by a reinforcing inlet nozzle welded between the feedwater pipe and the wall of the vessel. Feedwater is fed from the feedwater pipe through the nozzle by means of a section of pipe called a thermal sleeve, the upstream end of which is inserted in the nozzle and the downstream end of which is connected to feedwater distribution means such as sparger having a plurality of relatively small outlets or nozzles by which the cool feedwater is mixed with the hot water in the vessel. Prior thermal sleeve arrangements are shown, for example, in Japanese Pat. No. SHO-52-25518.
In a number of instances after extended use, cracks have been discovered along the inner surface of the inlet nozzle. As explained in greater detail hereinafter, these cracks are believed to be the result of thermal cycling of the inner portion of the nozzle by alternate exposure to the hot water in the vessel and to the relatively cooler feedwater or water cooled by the feedwaer.
An object of this invention is to provide a thermal isolator arrangement which substantially eliminates thermal cycling of the inner portion of the inlet nozzle.